# Mosfet amplifier issue with source current

#### captoro

Joined Jun 21, 2009
206
Hello,

I have uploaded a screen shot of my circuit. In this configuration I get a good voltage output but the current on R2 is 10x the current on the load L1.
I did increase the resistance on R3 but then my voltage does not decrease a lot, it varies between 45v to 42 volts on the output.
How do I decrease the current on R2, the wattage is going to blow my MOSFET. is there a configuration where that is possible? If I could have equal current on R1 and R3 that would be perfect.
I don't want to change MOSFET by the way.

Ken

#### PaulEE

Joined Dec 23, 2011
474
What exactly are you aiming to do with this here circuit?

Paul
KI5VNH

#### captoro

Joined Jun 21, 2009
206
What exactly are you aiming to do with this here circuit?

Paul
KI5VNH
The load is a coil ( antenna).

#### captoro

Joined Jun 21, 2009
206
By putting the coil on the drain and varying the resistor in series, I am able to get a nice waveform output AND control the current. So i think I got it. unless someone has a better idea.

#### the electrical doctor

Joined Dec 30, 2022
1
Hello,

I have uploaded a screen shot of my circuit. In this configuration I get a good voltage output but the current on R2 is 10x the current on the load L1.
I did increase the resistance on R3 but then my voltage does not decrease a lot, it varies between 45v to 42 volts on the output.
How do I decrease the current on R2, the wattage is going to blow my MOSFET. is there a configuration where that is possible? If I could have equal current on R1 and R3 that would be perfect.
I don't want to change MOSFET by the way.

Ken
the current in r2 is a function of the amplitude of the drive voltage
in your application you are trying to drive the mosfet to saturation current which appears to be happening
because the mosfet current is turned on and off the wattage of r2 does not follow ohms law
it is the average of the on time power time divided bt total time
if on time equals off time then power consumed equals half on power
the mosfet has 3 conditions of concern
the drain to source voltage during off time must not exceed vds max which is your supply voltage
temperature of junction at on time which is controlled by heatsink
ids max which is controlled by amplitude of input signal
as it is being pulsed ids max is the max pulsed current which is about 4 times the unpulsed current and controlled by heat sink the current in r3 is basically controlled by the impedance of the choke
the circuit would be considered as not efficient
provided the mosfet is within the limits described and a good heatsink the mosfet will survive

#### BobTPH

Joined Jun 5, 2013
6,293
If the idea is to pulse current through the inductor, place it where R2 is.

What you have now, is current flows through R2, the inductor, and R1 when the MOSFET is off, and is shorted to ground when the MOSFET is on.

#### captoro

Joined Jun 21, 2009
206
Hello Bob,

this is what I have described in my last post. Here is a screen shot.

#### BobTPH

Joined Jun 5, 2013
6,293